Inkjet printers are now very common and affordable and allow one to obtain decent print quality. They are used in home printing, office printing and commercial printing. In inkjet printers, print heads are used to eject ink droplets very accurately to place them on a desired location on a medium. The print head normally comprises a large number of nozzles, often, more than 400 nozzles. As a general rule, the larger the number of nozzles, the greater is the improvement of the print quality and speed. Frequently, the nozzles become blocked because of the usage of pigmented inks or inks containing particles. Sometimes, one or more nozzle orifices may contain dried ink and fresh ink cannot be ejected. One result of this condition is the formation of streaks, which lead to poor print quality.
NIR absorbing dyes are added to the ink to monitor the condition of the nozzles. NIR absorbing dyes may be employed by extending the conjugation so that the absorption can be shifted to the range of 680 to 850 nm range. Some examples of compounds that show near infrared absorption are cyanine dyes. However, cyanine dyes tend to be or become unstable in ink.